When observed
This is one of the aShTa mahA vartams told in ska.ndha purANam. It is observed on the full moon day when the sun is shining in the vruchchika rAsi that is in the month of kArthikai(Mid Nov to Mid Dec).
Way of observing
Getting up early in the morning on that full moon day, finishing the daily duties, one worships the form of sri umA mahesvara made of gold or silver, with various sacrad materials of worship and offerings. Later going to a temple one anoints the pArvati-paramEsvara with panychAmrutam (fruits mixed with sugar) and other substances, salute with offerings and tAmbUlam, pray the Lord making the gestures of salutations. Coming back home donating to the love hearted devotees of Lord siva, one eats with them only once that day or may eat fruits and sweets along with them. The idol of the Lord umA mahesvara is given to a sivanyAni.
One who observes this vrata sincerely, would get the material benefits of the world. Afterwards would get the liberation too. A sightless muni called dhiraNabin^dhu observing this vrata got beautiful eyes. A lady by the glory of this vratam revived her dead husband, gave birth to a child and got to reach sivalOka. viSNu following this vratam in kR^iShNAvatAram got many children. braHmA got married to gAyatri, sAvitri, indra got the son jayantan, vaShiShTar received kAmadEnu for doing his yagnya, gautamar got the son sadAnanta mahaR^iShi, janaka got the daughter sIta. A dumb minded person called sudharman, mocked by the people about his stupidity, observed this vratam in devotion. By the Lord's blessing he got the grace of goddess sarasvati and he became a pundit. The king sindhusenan was chased away by the enemies. He observed this vratam as per the advise of saint agastiyar. On the Lord's order kubera won back the kingdom for him and handed over to him. This vratam gets to the observer the wealths and takes to the path of liberation.